A stent delivery system can include a core assembly sized for insertion into a corporeal lumen and configured for advancing a stent toward a treatment location in the corporeal lumen. The core assembly can include a proximal restraint having a bumper section, a distal section distal to the bumper section, a lumen extending through the bumper and distal sections, and an undercut section including a recess at least partially surrounding the lumen. The recess can abut and/or be formed in the bumper and/or distal sections.